Streptococcus pneumoniae (“S.p.”) is an important human pathogen that colonizes the upper respiratory tract. It is a major cause of morbidity and mortality worldwide (Hollingshead & Briles, “Streptococcus pneumoniae: New Tools for an Old Pathogen,” Curr. Opin. Microbiol. 4:71-7 (2001)). It causes invasive diseases such as pneumonia, meningitis and bacteraemia, as well as otitis media and sinusitis. Despite the widespread use of antibiotics, the mortality rate from severe S. pneumoniae pneumonia remains highest during the first 48 hours of hospitalization and has not decreased appreciably over the past 30 years (Brandenburg et al., “Clinical Presentation, Processes and Outcomes of Care for Patients with Pneumococcal Pneumonia,” J. Gen. Intern. Med. 15:638-46 (2000); Hollingshead & Briles, “Streptococcus pneumoniae: New Tools for an Old Pathogen,” Curr. Opin. Microbiol. 4:71-7 (2001)). Moreover, successful treatment of S.p.-induced pneumonia has been further hampered by the increasing prevalence of antibiotic resistant strains worldwide. The earliest stage is seldom recognized and is most likely to be found in patients who die after illness lasting only a short time period because of the very rapid progress of the disease in some of the infected individuals (Loeschcke, Beitr. Path. Anat. 86:201 (1931)). The molecular mechanism underlying the high early mortality, however, remains unknown (Grigoryev et al., “Science Review: Searching for Gene Candidates in Acute Lung Injury,” Crit. Care 8:440-7 (2004); Kadioglu & Andrew, “The Innate Immune Response to Pneumococcal Lung Infection: The Untold Story,” Trends Immunol. 25:143-9 (2004)).
Histologically, the initial phase of pneumococcal pneumonia is characterized by acute lung injury (“ALI”). ALI is defined as an inflammatory disorder of the lung, which is characterized by hypoxemia, diffuse bilateral infiltrates on chest radiograph, and absence of atrial hypertension. Although numerous bacteria are present in an actively spreading lesion, few inflammatory cells are seen in serous exudates of these lesions because leukocytes have not had time to reach the alveoli in the advancing edema zone, suggesting that alveolar epithelial cell injury may be caused directly by pneumococcal toxins rather than by leukocytes themselves or their products (Hasleton, “Pulmonary Bacterial Infection,” in SPENCER'S PATHOLOGY OF THE LUNG 189-256 (Philip S. Hasleton ed., 5th ed. 1996); Tuomanen et al., “Pathogenesis of Pneumococcal Infection,” N. Engl. J. Med. 332:1280-4 (1995); Wood, W. B., “Studies on the Mechanism of Recovery in Pneumococcal Pneumonia: I. The Action of Type Specific Antibody Upon the Pulmonary Lesion of Experimental Pneumonia,” J. Exp. Med. 73:201-22 (1941)).
Despite the importance of pneumococcal diseases, little is known about the molecular mechanisms by which S.p.-induced lethality is regulated (Tuomanen et al., “Pathogenesis of Pneumococcal Infection,” N. Engl. J. Med. 332:1280-4 (1995); Kadioglu & Andrew, “The Innate Immune Response to Pneumococcal Lung Infection: The Untold Story,” Trends Immunol. 25:143-9 (2004)). Among a variety of virulence factors that have been identified, pneumolysin, a 53 kDa protein produced by virtually all clinical isolates of S. pneumoniae, plays an important role in mortality associated with S. pneumoniae infections (Cockeran et al., “The Role of Pneumolysin in the Pathogenesis of Streptococcus pneumoniae Infection,” Curr. Opin. Infect. Dis. 15:235-9 (2002)) by inducing important pathological processes, including hemorrhage, mainly due to its well-established hemolytic cytotoxicity. Pneumolysin is located in the cytoplasm, but is released when pneumo cocci undergo spontaneous autolysis (Cockeran et al., “The Role of Pneumolysin in the Pathogenesis of Streptococcus pneumoniae Infection,” Curr. Opin. Infect. Dis. 15:235-9 (2002); Tuomanen et al., “Pathogenesis of Pneumococcal Infection,” N. Engl. J. Med. 332:1280-4 (1995); Paton, “The Contribution of Pneumolysin to the Pathogenicity of Streptococcus pneumoniae,” Trends Microbiol. 4:103-6 (1996); Jedrzejas, “Pneumococcal Virulence Factors: Structure and Function,” Microbiol. Mol. Biol. Rev. 65(2):187-207 (2001); Paton et al., “Molecular Analysis of the Pathogenicity of Streptococcus pneumoniae: The Role of Pneumococcal Proteins,” Annu. Rev. Microbiol. 47: 89-115 (1993)). Pneumolysin is classically defined as a pore-forming hemolysin and is able to lyse the plasma membrane of virtually any mammalian cell.
There has been growing evidence indicating that pneumolysin plays an important role in inducing acute lung hemorrhage and mortality, especially during the early stages of lethal S.p. infections. Pathologically, the initial phase of S.p.-induced pneumonia is mainly characterized by pulmonary alveolar hemorrhage, edema, and intra-alveolar bacterial multiplication but minimal numbers of inflammatory cells, suggesting that pneumolysin is capable of disrupting the normal alveolar-capillary barrier (Rubins & Janoff, “Pneumolysin: A Multifunctional Pneumococcal Virulence Factor,” J. Lab. Clin. Med. 131:21-7 (1998); Wood, W. B., “Studies on the Mechanism of Recovery in Pneumococcal Pneumonia: I. The Action of Type Specific Antibody Upon the Pulmonary Lesion of Experimental Pneumonia,” J. Exp. Med. 73:201-22 (1941)). Indeed, pneumolysin has been shown to be cytotoxic to alveolar epithelial cells and pulmonary endothelial cells in vitro (Rubins et al., “Toxicity of Pneumolysin to Pulmonary Endothelial Cells in Vitro. Infect. Immun. 60:1740-6 (1992); Rubins et al., “Toxicity of Pneumolysin to Pulmonary Alveolar Epithelial Cells,” Infect. Immun. 61:1352-8 (1993)) and disrupts the alveolar-capillary barrier in isolated perfused lungs (Rubins et al., “Toxicity of Pneumolysin to Pulmonary Alveolar Epithelial Cells,” Infect. Immun. 61:1352-8 (1993)). Moreover, histopathological change of pneumococcal pneumonia was reproduced with pneumolysin in vivo (Maus et al., “Pneumolysin-induced Lung Injury Is Independent of Leukocyte Trafficking into the Alveolar Space,” J. Immunol. 173:1307-12 (2004)). Electron microscopy revealed that instilled pneumolysin caused widespread lung injury. Direct cytotoxic effect of pneumolysin to the alveolar epithelium, as well as to the pulmonary endothelium, may produce alveolar flooding and hemorrhage during the earliest stages of pneumococcal pneumonia. The resulting serous exudates may in turn promote the rapid multiplication of S. pneumoniae within the alveoli. The lesion progresses to the state known as red hepatization, which results from leakage of erythrocytes into the alveoli (Rubins et al., “Toxicity of Pneumolysin to Pulmonary Alveolar Epithelial Cells,” Infect. Immun. 61:1352-8 (1993)).
Despite the availability of antibiotic and intensive supportive therapy, this early mortality has not been significantly reduced over the past 30 years. Moreover, successful treatment of S.p.-induced pneumonia has been further hampered by the increasing prevalence of antibiotic resistant strains worldwide. Therefore, currently there is an urgent need for developing novel therapeutic strategies for controlling S.p. pneumolysin-induced lung hemorrhage and reducing mortality.
The present invention is directed to overcoming these and other deficiencies in the art.